Success!

Well Sherlock fans, it's time to prepare for your heart to be inevitably broken. Not only will we be waiting until 2016 for the 4th season of Sherlock, but we can expect some sort of tragedy. Mark Gatiss (co-creator of the hit series) spoke with Radio Times on Thursday and explained, "You can always expect tragedy as well as adventure, that’s just how it goes". It's a good thing we have some time to prepare our hearts for the expected barrage of sadness to come.
According to The Hollywood Reporter, fans have been speculating that the tragedy will be the death of Mary, John's wife, because the death is canon in Arthur Conan Doyle's Sherlock book series. Martin Freeman, who plays the beloved Dr. John Watson, has himself predicted that Mary will meet her demise at one point or another. “While we play fast and loose with the original stories, we generally follow the trajectory of what Conan Doyle did. … So [John] gets married, and then Mary dies — so at some point presumably she’ll die,” claimed Freeman during his talk with the Daily Telegraph.
Gatiss indirectly responded to the assumptions of Mary's death by stating, "Just because it’s in the stories doesn't mean it’ll happen in the series because there’s an awful lot of changes and an awful lot of places to go and things to do." Gatiss goes on to say that although Doyle is their so-called "god", they're more than willing to stray from their source material. The addition of Sherlock's parents is one such example because they have not been seen in any adaptation thus far, so refreshing surprises and twists are to be expected.

Courtesy of PBS

I believe that Mary will eventually meet her end but I don't think it'll be this season. It just appears to be too soon to kill off a character who was only introduced last season. But then again (in a sense) it'll be a bigger shock to have Mary abruptly taken away from John. I can see Mary being killed in the finale leaving us with a distraught john, ultimately putting Sherlock in the position of consoling his best friend and taking care of him (a bit of a role reversal). However, with Mary being pregnant, her possible death will have John being a single parent which will leave Sherlock with the responsibility of helping John raise the child.
Theories are brewing in my head (seriously, I could go on and on about my speculations) but we'll all have to wait until 2016 to get a definite answer.
The 2015 Sherlock Christmas special begins production in January 2015 (hooray!) whereas the 4th season is expected to start filming later in 2015, to premiere in 2016.
Let us know in the comments below what your speculations are about the tragedy that is set to occur in the new season of Sherlock!